MEMS are devices which integrate small mechanical devices with semiconductors to form sensors (temperature, pressure, gas, moisture, and motion), accelerometers, valves, gears, actuators, and micromirrors. MEMS devices are often required to survive in hostile or toxic environments as medical, military, space, and other applications. They typically require hermetically sealed packages for proper operation. It may be difficult to establish whether a package is hermetically sealed. It is possible that a small leak may exist which is noticeable only after a long period of operation. A leaky cavity can render the device inoperable when the vacuum or operating environment no longer exists.
Particulate contamination within a cavity can reduce mechanical performance and moisture can cause changes in adhesion and cohesion. The effect of parameters such as temperature on moisture can vary from formation of ice at low temperatures to steam at high temperatures within a cavity.
Devices with cavities may require a vacuum or an atmosphere for operation. A leak detection test may be limited by its ability to function in only a vacuum or an atmosphere. It is also possible that a leak detection test may be limited by its ability to function only in singulated device form or only in wafer form.
Leak detection of cavities becomes more difficult as cavities become smaller. In MEMS devices using semiconductor substrates, leak detection errors may occur due to wafer bow and variation in substrate thickness. The need for special gases, liquids, or materials adds difficulty, complexity, and cost to the leak detection test.
A digital micromirror device (DMD), such as a Texas Instruments DLP® micromirror device, is typically placed in a hermetically sealed package prior to singulation. Micromirrors are sensitive to the environment. For example, if moisture is present within a micromirror cavity, then stiction or the ability of the mirror to tilt may deteriorate and the micromirror may require additional current or voltage to move between an “ON” state and an “OFF” state. It is also possible that the micromirror will be unable to move from a fixed position and form a defect in the micromirror array.
One method of detecting a leak in a device cavity involves flowing a reverse current through a PN junction diode. The test checks for the presence of moisture and humidity within the sealed cavity using changes in electrical parameters.
Another leak detection method involves introduction of a gas into a packaged device, evacuation of the cavity by vacuum suction, and a scan of the device for escaped gases using a spectrometer.
Yet another method of cavity leak detection comprises an oscillating structure where changes in a quality value are determined by applying pressure to the outside of a packaged sensor. Changes in oscillation are tied to changes in quality value.
Some methods for cavity leak detection may require special features such as optically clear windows, infra-red clear zones, or the use of radioactive gases. Methods requiring special internal structures or special gases or liquids for testing add cost and complexity to the test. For example, a method for leak testing using Krypton-85 radioisotope is sensitive and able to measure both gross and fine leak rates. It is a rapid technique but it has the disadvantage of requiring a radioactive gas, Krypton-85. The use of radioactive gases requires specific safety procedures which also increase cost and complexity.
FIG. 1A (Prior art) shows a wafer substrate 100 with devices 102 formed in a grid pattern. The grid is defined by scribe streets or scribe lines 104.
The substrate 100 is attached to a second substrate for support, using a bonding material. The two substrates are sawn prior to packaging the devices 102. The devices 102 are singulated into individual die by fracturing the substrates along sawn scribe lines.
FIG. 1B (Prior art) shows a cross-section of the wafer substrate 100 with devices attached to a support wafer 106. The substrate 100 is attached to the substrate 106 using a bonding adhesive 108. Devices 102 have been formed within the surface 110 of the top substrate 100. A gap is formed between substrate 100 and substrate 108 creating a small cavity 112 which is bounded by the bonding adhesive 108.